1. Field of the Invention
This invention relates to a devolatilization apparatus to be provided for a first step of equipment for manufacturing thermoplastic resins such as styrene polymers to remove continuously and efficiently unreacted monomer, solvent and like volatile substances from a liquid composition containing polymer and volatile constituents (hereinafter referred to as polymer solution) and a method for devolatilization using the same apparatus.
2. Discussion of the Prior Art
Usually, when causing bulk or solution polymerization of styrene either alone or together with other copolymerizable monomers, the polymerization reaction is discontinued when polymerization of 40 to 90% by weight of monomer has occurred, and unreacted monomer remaining in the resultant polymer solution, and solvent and like volatile substance also remaining in the solution in the case of the solution polymerization, is separated. Particularly, low residual volatile component content is required in case of styrene polymer used for food containers or the like. When performing devolatilization operation, it is important for obtaining satisfactory quality of polymer to pay attention lest the polymerization proceeds excessively and also lest the polymer solution should be exposed to a high temperature for long time, thus avoiding quality deterioration due to otherwise possible generation of low molecular weight polymers or crosslinking with rubber phase in case where a rubber component is contained.
An apparatus for removing volatile components of a styrene-based polymer solution usually includes a heat exchanger for giving the polymer solution an amount of heat necessary for the evaporation of the volatile components and also heating the polymer solution after the evaporation of the volatile components to maintain adequate fluidity of polymer, a devolatilizer for causing evaporation of volatile components contained in the polymer solution and vacuum means. To obtain high quality polymer, i.e., to reduce quality degradation of polymer under a high temperature condition, it is desired to minimize the distance between the heat exchanger and devolatilizer, increase the surface area of the polymer solution per unit volume in the devolatilizer and increase the amount of evaporation of the volatile components. As apparatus with a minimum distance between the heat exchanger and devolatilizer, U.S. Pat. Nos. 4,537,795 and 4,699,976 disclose structures, in which a vertical multi-tube heat exchanger for heating polymer solution taken out from a polymerization process is disposed on a devolatilizer to permit the polymer solution having passed through the heat exchanger to flow down immediately into the devolatilizer while forming bubbles. The prior arts noted above also propose to add, in the use of the apparatus, a bubble-forming agent such as water or alcohol to polymer solution for improving the devolatilization efficiency.
However, in the disclosed apparatus the evaporation surface area of the polymer solution is limited by the number and diameter of the heat exchanger tubes. This means that a limitation is imposed on the reduction of the amount of volatile components remaining in the polymer solution after the devolatilization treatment. If it is intended to elevate the temperature of the polymer solution for reducing the content of the residual volatile components, low molecular weight polymer will be produced to degrade the quality of polymer. It is proposed to perform devolatilization in two or three steps in order to solve the above problem. With this method, however, sufficient effects cannot be obtained.
An object of the invention is to provide efficient apparatus and method for devolatilizing polymer solution, i.e., liquid composition containing polymer and volatile constituents, which permit reduction of the concentration of residual volatile components in the polymer solution to 300 ppm or below without need of any great equipment investment or energy consumption.